Single Column Stripping and Drying Process

ABSTRACT

Organic materials are stripped and dried in a single column having two contact zones. A stripping gas is introduced into an upper contact zone and flows through the organic material in that zone. A drying gas is introduced into a lower contact zone. The drying gas contacts the organic material in both the upper and lower contact zones, and is removed from the top of the column together with the stripping gas. This process permits very efficiently removal of volatile organic compounds as well as efficient drying, while requiring on low levels of the stripping and drying gasses.

The present invention relates to a process for stripping impurities fromorganic compounds and organic polymers such as poly(alkylene oxide)polymers.

Many organic compounds and polymers are manufactured using processesthat leave them with some level of impurities and/or moisture. It isoften necessary to remove those impurities and that moisture to very lowlevels, so that the product is suitable for use in some downstreamoperation. There are many instances of this in the chemical industry.One prominent example is in manufacturing poly(alkylene oxide) polymers;the crude polyols often contain significant amounts of volatile aldehydeimpurities which, among other things, impart a noxious odor to thepolyols and to products such as polyurethane foam that are made from thepolyols. In addition, the polyether polyols often contain up to 5% or soby weight of water, which needs to be reduced to much lower levelsbefore the polyols can be used in polyurethane manufacturing. A relatedmanufacturing process is the production of so-called polymer polyols bythe polymerization of monomers such as styrene and/or acrylonitrile inthe presence of a poly(alkylene oxide) polyol. The polymer polyolproducts generally contain significant quantities of volatile organicimpurities, especially unreacted monomers, which need to be removed fromthe product before it can be used.

Stripping methods are commonly used to remove volatile impurities fromorganic compounds to very low levels. One common approach to strippingis to pass the organic compound downwardly through a stripping column,which is often operated at subatmospheric pressures. A stripping gas,usually steam, is passed upwardly through the column, countercurrent tothe organic compound, and carries volatile organic impurities with itout of the top of the column.

Stripping processes for removing volatile impurities from polyetherpolyols are described, for example, in U.S. Pat. No. 6,060,627 and USPublished Patent Application Nos. 2008/0033139 and 2008/0033214. In U.S.Pat. No. 6,060,627, a propoxylated glycerin containing water, allylalcohol and propylene glycol allyl ethers is sent to a reboilerevaporator to remove the bulk of the water, and then vacuum strippedwith steam as the stripping gas. The weight ratio of steam to polyetherpolyol mixture being treated is about 0.0287:1. In US 2008/0033139, atwo-column stripping apparatus is described for stripping residualmonomers from a polymer polyol product. The working examples of thatapplication describe very high steam to polyol ratios (0.2:1 by weight).These high levels of steam are described as reducing the levels ofresidual monomer and isopropanol from around 12,000 to 13,000 ppm to aslow as 1 ppm each, but more typical results are in the range of 10 to 20parts per million. In addition, a specific impurity, a “recombinationproduct” remains in the polyol at the level of hundreds of parts permillion, and water content is not reduced or is even increased.

US 2008/0033214 describes a stripping process for polyether polyolswhich, prior to treatment, are said to have at most 100 parts permillion volatile organic compounds. The stripping process removes theseonly to the 20 ppm level, which is said to be enough to reduce odor butis nonetheless a significant loading. The weight ratio of stripping gasto polyol is said to be from 0.01 to 0.05. The example describesstripping using nitrogen as a stripping agent.

Drying also can be performed in a column, by passing a drying gasthrough the column countercurrent to the flow of the organic material.

It is desirable to develop a stripping and drying process whicheffectively removes volatile organic impurities to very low levels, suchas 10 ppm or below, and which also removes water to low levels, such asto 200 ppm or below, and which does so efficiently and inexpensively.

This invention is such a process. This invention is a process forstripping and drying a starting organic material, comprising:

a) introducing the starting organic material into an upper contact zoneof a column that has an upper contact zone and a lower contact zone, andallowing the starting organic material to travel downwardly through thecolumn, through the upper contact zone and then through the lowercontact zone to a bottom section of the column;

b) introducing a stripping gas into the upper contact zone of the columnand above the lower contact zone of the column such that the strippinggas moves upwardly through the upper contact zone, where it contacts thestarting organic material and removes impurities therefrom, and then toa top section of the column;

c) introducing a drying gas into the lower contact zone of the columnsuch that the drying gas moves upwardly through the lower contact zonewherein contacts a portion of the organic material and removes moisturefrom therefrom, then moves upwardly through the upper contact zone whereit contacts another portion of organic material, and then moves to a topsection of the column;

d) removing the stripping gas and the drying gas from the top section ofthe column above the upper contact zone; and

e) removing the stripped and dried organic material from the bottomsection of the column below the lower contact zone.

The stripping process of this invention offers several potentialadvantages over previous approaches in which the stripping and dryingsteps are conducted separately. Costs for equipment are lower, becausethe stripping and drying are conducted in a single column instead ofmultiple columns. Volatile organic impurities often can be reduced tovery low levels using surprisingly small amounts of stripping gas. Thisresults in reduced costs for those materials and fewer costs associatedwith handling the larger volumes of gases. When the stripping gas issteam, as is preferred, the invention also offers advantages in energycosts (because less steam needs to be produced) and in wastewatertreatment costs (for example, to dispose of condensed steam and volatileorganic compounds entrained therein). Because the drying gas passesthrough the upper contact zone and contacts the organic material there,it contributes to the stripping that occurs in the upper contact zoneand thus reduces the amount of stripping gas that is needed. Thisreduces the size and cost of the vacuum system required to operate thecolumn at reduced pressure.

The process is well-adapted for continuous operation, which againreduces costs and eliminates in some cases the need for storing theorganic material prior to the stripping and drying process. Eliminatingor reducing the storage of the material can in some instances, as in thecase when the organic material is a polyether, reduce or eliminate theneed to add stabilizers such as antioxidants, or to make pH adjustmentsto the material, or to conduct other pretreatments of the material.

The FIGURE is a schematic drawing of the process of the invention.

Turning to the FIGURE, stripping column 9 includes liquid inlet 1,through which the starting organic polymer or compound containingvolatile organic impurities is introduced into column 9. The startingorganic material, upon entering column 9, flows downwardly through uppercontact zone 2 of column 9. Stripping gas is introduced into uppercontact zone 2 column 9 through stripping gas inlet 5. Stripping gasinlet 5 is below liquid inlet 1, where the starting organic material isintroduced into the column, and above lower contact zone 3. Generally,the location of stripping gas inlet 5 will demark the division betweenupper contact zone 2 and lower contact zone 3 of column 9.

Stripping gas introduced into column 9 travels upwardly, passing throughupper contact zone 2 where it contacts starting organic compound orpolymer and at least partially strips volatile (relative to the organicmaterial) impurities from organic material.

The stripping gas with entrained impurities then passes upwardly to topsection 10 of column 9, where it exits column 9 through gas outlet 4.Top section 10 is generally the portion of column 9 above liquid inlet1, where the gasses can accumulate to be removed from column 9 throughgas outlet 4.

The organic material, having passed downwardly through upper contactzone 2, then enters lower contact zone 3 of column 9. A drying gas isintroduced via drying gas inlet 6 into lower contact zone 3, where itcontacts the organic material and removes moisture therefrom. Generally,the location of drying gas inlet 6 will demark the division betweenlower contact zone 3 and bottom section 8 of column 9. The dried organicmaterial then flows to a bottom section 8 of column 9 where it collectsbelow drying gas inlet 6 and from where it is removed from column 9 viaoutlet 7. After passing through lower contact zone 3, the drying gaspasses upwardly through column 9 into upper contact zone 2, where itcontacts more of the organic material, at the same time as the organicmaterial contacts the stripping gas. The drying gas is believed tocontribute to the stripping operation that occurs in upper contact zone2. The drying gas then passes into top section 10 and is removedtogether with the stripping gas through gas outlet 4.

Apart from having the specific features described above (i.e., thevarious inlets and outlets and upper and lower contact zones), strippingcolumn 9 does not need to have any special construction or design. Itmay be constructed of any suitable material, taking into account theoperating conditions (mainly temperature and pressure), the particularorganic material, the particular stripping gas and the particular dryinggas. Various grades of steel or aluminum are generally suitable.

Stripping column 9 will generally contain a packing in at least uppercontact zone 2 and lower contact zone 3. The packing serves the purposesof distributing the organic material more uniformly across thecross-section of the column, increasing the residence time of theorganic material within the column, increasing the surface area of theorganic material to facilitate better contact with the stripping anddrying gases, and of facilitating heat transfer if needed. A widevariety of porous materials are suitable as the packing, includingmeshes, wools, fibers, a series of porous plates, beads or otherparticulates, various types of structured packings, trays and the like.As before, the material of construction is not considered to be criticaland will be selected taking the operating conditions and the particularorganic material, stripping gas and drying gas into account. Metal andceramic packings are generally preferred. If metal packings are used,the metal should be resistant to chemical attack from water and/or otherchemical species.

Stripping column 9 may also contain one or more supports for holding thepacking material in place within the column.

The packing in stripping column 9 may have a specific surface area offrom 150 to 500 m² per cubic meter. A preferred specific surface area isfrom 230 to 450 m²/m³.

Stripping column 9 may have a jacket for applying a heating or coolingmedium such as steam to the outside of the column to provide fortemperature control. Other heating and cooling devices may be present inaddition to or in place of such a jacket. Stripping column 9 will alsogenerally contain means for distributing the stripping gas supplied bystripping gas inlet 5 across the cross-section of the column. Therefore,stripping gas inlet 5 will generally be in fluid communication with astripping gas distribution means, through which stripping gas isreceived from stripping gas inlet 5 and introduced into stripping column9. The stripping gas distribution means preferably introduces thestripping gas into column 9 at multiple locations across itscross-section. The stripping gas distribution means may include, forexample, one or more distribution plates, spargers, bubbles, jets,nozzles or similar apparatus.

Similarly, drying gas inlet 6 will also generally be in fluidcommunication with means for distributing the drying gas across thecross-section of stripping column 9. Suitable designs for such dryinggas distribution means include those mentioned above with regard to thestripping gas distribution means.

Stripping column 9 may contain means for distributing the organicmaterial introduced through inlet 1 across the cross-section of thecolumn. Again, specific apparatus as are mentioned above with regard tothe stripping gas distribution means are entirely suitable.

Gas outlet 4 is preferably in fluid communication with a gas removalmeans for withdrawing the stripping and drying gases from column 9, and,if column 9 is to be operated under subatmospheric pressure, for pullinga vacuum onto column 9. The gas removal means may be of any suitabledesign. Mechanical devices such as simple fans or blowers and vacuumpumps of various designs are suitable. If the stripping gas is steam orother easily condensable gas, the gas removal means may consist of orinclude one or more condensers, which condense the gas and therebyproduce a vacuum in the system. Combinations of these approaches can beused.

Outlet 7 also is preferably in fluid communication with a pumping meansfor withdrawing the stripped and dried organic material from the column.The pumping means is any sort of device for moving a liquid through aconduit. Various types of pumps and impellers are suitable. The designof the pumping means is not considered to be critical to the invention.

The organic material is any organic compound or mixture of organiccompounds, which is a liquid under the conditions at which the strippingcolumn is operated. A mixture of organic compounds may include asolution of one material in a solvent, which solvent may be a processsolvent from some upstream step, or one which is used to dissolve orreduce the viscosity of another organic material for processing throughthe stripping column. The organic material, prior to treatment accordingto this invention, will generally contain one or more impurities whichare more volatile than the organic material (and more volatile than anysolvent which is needed to maintain the organic material as a liquidthrough the stripping and drying process) and which can be removed bystripping. The organic material may contain moisture, i.e., some smallamount of water, prior to being introduced into stripping column 9. Thisis the usual case when the stripping gas is something other than steam.When steam is used as the stripping gas, the organic material willusually absorb some moisture from the steam in upper contact zone 2,which moisture is then removed through contact with the drying gas inlower contact zone 3.

The organic material may include or consist of (apart from impuritiesand moisture) one or more organic polymers, which may or may not bedissolved in some solvent.

The starting organic material may contain, for example, from 10 to20,000 parts per million by weight (ppm) of volatile organic impurities.Greater levels of organic impurities are difficult to remove to very lowlevels in a stripping process. A preferred level of volatile organicimpurities in the starting organic material is from 50 to 5,000 ppm anda more preferred level is from 50 to 2,000 ppm. The starting organicmaterial may contain up to 50,000 ppm of water, up to 10,000 ppm water,up to 5,000 ppm of water, or up to 2,500 ppm of water.

A preferred organic material is a crude liquid polyether having amolecular weight of from about 500 to 10,000, which is the product of analkylene oxide polymerization reaction. The alkylene oxide may be, forexample, ethylene oxide, propylene oxide, butylenes oxide, 1,2-hexaneoxide, tetrabutylene oxide, styrene oxide, or a mixture of any two ormore thereof. These oxides can be polymerized in an anionicpolymerization process in the presence of an initiator compound and ofan alkali metal hydroxide catalyst. Alternatively, the oxides can bepolymerized, again in the presence of an initiator compound, using adouble metal cyanide catalyst complex. The crude liquid polyether willgenerally be terminated in hydroxyl groups, and may have, for example,from 1 to 12 hydroxyl groups per molecule. Crude liquid polyethersmanufactured in these ways typically contain a number of organicimpurities, including unreacted oxides, various aldehyde by-products andthe like, which are more volatile than the liquid polyether and aresusceptible to being removed in a stripping process. Polyethers madeusing an anionic polymerization process are often in addition treated toneutralize and remove the catalyst residues and for that reason oftencontain some amount of entrained moisture.

Another preferred organic material is a crude polymer polyol which isobtained from a polymer polyol process in which one or moreethylenically unsaturated monomers are polymerized in the presence of apolyether polyol. Such crude polymer polyol products tend to contain upto 20,000 ppm, preferably up to 10,000 of unreacted monomers, chaintransfer agents, free radical initiators and/or decomposition residuesof these.

The stripping gas is selected such that, under the conditions used inthe stripping column, it exists in the gas phase and does not react withthe organic material. The stripping gas also preferably has lowsolubility in the organic material. A preferred stripping gas is easilycondensable, as this allows a vacuum to be pulled in the strippingcolumn by condensing the stripping gas that has been removed from thecolumn. Superheated steam is an especially preferred stripping gas. By“superheated” it is meant that the steam is at a temperature above itsdew point under the conditions at which the column is operated. Mixturesof gases can be used, such as, for example, a mixture of superheatedsteam and air, or of superheated steam and an inert gas such as nitrogenor argon. The amount of stripping gas may range from 0.0002 to 0.1 kg ormore of stripping gas per kg of organic material. A preferred amount isfrom 0.0002 to 0.03 kg per kg of organic material and a more preferredamount is from 0.0002 to 0.02 kg per kg of organic material.

The drying gas is also selected such that, under the conditions used inthe stripping column, it exists in the gas phase and does not react withthe organic material, and also preferably has low solubility in theorganic material. The drying gas should contain no more than 100,preferably no more than 50 ppm of water. Dry air is a useful strippinggas, but inert gasses such as argon and especially nitrogen arepreferred. The amount of drying gas may range from 0.0001 to 0.1 kg oforganic material. A preferred amount is from 0.0001 to 0.003 kg per kgof organic material.

The temperature and pressure conditions at which the stripping anddrying process is performed will of course depend on the particularorganic material, the nature of the organic impurities and theparticular stripping and drying gases that are used. The temperature andpressure in the stripping column are such that the organic material(apart from volatile organic impurities that are to be removed) remainsin the liquid state as it passes through the column, and such that thestripping and drying gases remain in the gaseous state as they pass upthrough the column. Subatmospheric pressures are preferred, tofacilitate removal of the volatile organic impurities and water from theorganic material. The pressure inside the stripping column may be, forexample, from 5 to 100 mbar or from 15 to 50 mbar. A suitabletemperature may be from 0 to 160° C. A preferred temperature, especiallyfor stripping polyethers is from 90 to 150° C., and a more preferredtemperature is from 120 to 140° C.

It is generally desirable to reduce the amount of organic impurities andmoisture in the treated organic material to very low levels, such as,for example, to 10 ppm or below in the case of organic impurities and to200 ppm or below in the case of residual moisture. It is more preferredto reduce the level of the volatile organic impurities to 1 ppm orbelow.

The following examples are provided to illustrate the invention, but notto limit the scope thereof. All parts and percentages are by weightunless otherwise indicated.

EXAMPLE 1

A single stripping column designed as shown in FIG. 1 is constructed. Acrude polyol containing 6,000 ppm water and 1,000 ppm volatile organicimpurities is passed through the column at 130° C. The column isoperated at 15 mbar. The stripping gas is superheated steam, and thedrying gas is nitrogen. Only 0.005 kg of steam per kg of crude polyol isneeded to reduce the amount of volatile organic impurities to 0.5 ppm.The amount of nitrogen drying gas is only 0.0004 kg per kg of crudepolyol; this amount is sufficient to reduce the water content to 100ppm. Therefore, with this invention, not only is steam consumption verylow but the total amount of stripping plus drying gas is also very lowand the water content is reduced dramatically in addition to thenear-complete removal of volatile organic compounds.

1. A process for stripping and drying a starting organic material,comprising: a) introducing the starting organic material into an uppercontact zone of a column that has an upper contact zone and a lowercontact zone, and allowing the starting organic material to traveldownwardly through the column, through the upper contact zone and thenthrough the lower contact zone to a bottom section of the column; b)introducing a stripping gas into the upper contact zone of the columnand above the lower contact zone of the column such that the strippinggas moves upwardly through the upper contact zone, where it contacts thestarting organic material and removes impurities therefrom, and then toa top section of the column; c) introducing a drying gas into the lowercontact zone of the column such that the drying gas moves upwardlythrough the lower contact zone where it contacts a portion of theorganic material and removes moisture from therefrom, then movesupwardly through the upper contact zone where it contacts anotherportion of organic material, and then moves to a top section of thecolumn; d) removing the stripping gas and the drying gas from the topsection of the column above the upper contact zone; and e) removing thestripped and dried organic material from the bottom section of thecolumn below the lower contact zone, wherein the starting organicmaterial is a liquid polyether having a molecular weight of from 500 to10,000 or crude polymer polyol, the starting organic material containsfrom 50 to 5,000 ppm of volatile organic impurities and up to 50,000 ppmof water, and the stripped and dried organic material contains up to 10ppm of volatile organic compounds and up to 200 ppm of water.
 2. Theprocess of claim 1 wherein the stripping gas is superheated steam. 3.The process of claim 1 wherein the drying gas contains no more than 50ppm moisture.
 4. The process of claim 3, wherein from 0.002 to 0.03parts by weight of stripping gas are passed through the column per partby weight of the organic material.
 5. The process of claim 4, whereinfrom 0.0001 to 0.1 parts by weight of drying gas are passed through thecolumn per part by weight of the organic material.
 6. The process ofclaim 5, wherein the column is operated at a pressure of from 5 to 100mbar.
 7. The process of claim 6, wherein the column is operated at atemperature of from 90 to 150° C. 8-10. (canceled)